Zinc electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings

ABSTRACT

The invention relates to an aqueous electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings and to a method for producing such an electrolyte. The electrolyte comprises (a) Zn 2+  in a concentration of 15 to 70 g/L; (b) Cl −  in a concentration of 100 to 200 g/L; (c) K +  and/or Na +  in a total concentration of 0.75 to 6.0 mol/L; (d) acetate in a concentration of 5.0 to 45 g/L; (e) glycine and/or alanine in a total concentration of 0.5 to 30 g/L; and (f) water. The electrolyte has a pH of 4.5 to 6.5. In a preferred variant, the electrolyte contains (g) nicotinic acid and/or (h) ethoxylated thiodiglycol. The invention also relates to a method for producing a component having a zinc coating, which uses the electrolyte.

TECHNICAL FIELD

The invention relates to an aqueous electrolyte devoid of boric acid andammonium for the electrodeposition of zinc coatings, to a method forproducing such an electrolyte and to a method for producing a componenthaving a zinc coating by way of electrodeposition from this electrolyte.

TECHNICAL BACKGROUND

Zinc coatings are used in many technical fields since they are able toprovide very good cathodic corrosion protection for components, inparticular those made of ferrous materials. The zinc coatings can beformed on the metal component by way of electrodeposition(electrochemical deposition) from a zinc electrolyte. The depositionpreferably occurs from weakly acidic electrolytes since these generallyfacilitate significantly higher deposition speeds than alkalineelectrolytes. A good deposition speed is important for a cost-effectivecoating of components, for example of accessory parts for automobilemanufacturing.

Generally, a good optical impression, i.e. a homogenous and glossysurface, of the zinc coating is desirable. Moreover, for many technicalapplications it is also customary to anneal the zinc coatings. Annealingdrives out the hydrogen absorbed during the electroplating process, inorder to counteract the risk of the component subsequently becomingembrittled. Particularly in the case of high-strength steel components,there is an increased risk of hydrogen-induced brittle fracture due tothe material. During the annealing process, it is desirable to avoidbubbles and other defects if at all possible, and to maintain a goodoptical impression.

Until now, the prior art for the deposition of zinc coatings has been touse a weakly acidic zinc electrolyte containing boric acid as a buffer.Boric acid is considered necessary to be able to work at high currentdensities (e.g. 6 to 8 A/dm²), which is desirable for a high depositionspeed. Boric acid prevents haphazard and dendritic growth of the zinclayer at high current densities, which is important for annealabilityand the optical properties of the zinc coating. If boric acid isomitted, it is generally the case that disordered, powdery, black zinclayers are deposited.

Boric acid also prevents the reduction of protons to hydrogen(2H⁺+2e⁻→H₂), which can occur at the cathode in addition to thereduction of zinc ions to zinc (Zn²⁺+2e⁻→Zn). The reduction of protonsincreases the pH in the surroundings of the cathode, since an excess ofnegatively charged hydroxide ions is generated. This leads to zinchydroxide precipitating directly on the cathode surface and can resultin coating defects.

Boric acid is also characterized by an affordable price, electrochemicalstability, process compatibility and unproblematic wastewater treatment.

However, boric acid and related compounds containing boron, such asdisodium tetraborate, diboron trioxide and tetraboron disodiumheptaoxide, are considered Category 1B reproductive toxicants(teratogenic) and on 18 Jun. 2010 they were added to the candidate listfor the REACH Regulation. After a thorough review and consideration ofall known data and facts, on 1 Jul. 2015 the European Chemicals Agency(ECHA) argued in favor of the inclusion of boric acid in Annex XIV ofthe REACH Regulation on substances requiring authorization and submitteda corresponding recommendation to the EU Commission. Although boric acidand the other boron compounds mentioned above meet the criteria forinclusion in Annex XIV of the REACH Regulation, the EU Commissiondecided not to classify these compounds as requiring authorization forthe time being (Regulation (EU) 2017/999, as published in the OfficialJournal of the European Union on 13 Jun. 2017).

Even though boric acid does not (yet) currently require authorization inthe EU, it is desirable for sustainability reasons and on health andsafety grounds to reduce the use of boric acid or to avoid italtogether.

Electrolytes containing zinc that are devoid of boric acid have beendescribed in the literature in isolated cases:

In a publication by J. Heber (Galvanotechnik, 2014, Vol. 105, 2150-2156)it was proposed that boric acid be replaced by acetic acid. It was notedin this publication that the use of acetic acid limits use at highcurrent densities. Moreover, replacing boric acid with acetic acidworsens the metal distribution on the substrate and reduces the cloudpoint (temperature at which the organic additives precipitate).Furthermore, a lower current yield is observed than with boric acid.

In EP 2 706 132 A1, the use of a weakly acidic zinc nickel electrolytedevoid of boric acid is described, which uses, in particular, a mixtureof adipic acid, succinic acid, glutaric acid, sulfosuccinic acid andpropionic acid to replace boric acid. In addition to these organic acidsused as buffer substances, stronger complexing agents (such as DETA orEDA) for the nickel also have to be added to a zinc-nickel electrolyteso that nickel can be sufficiently incorporated into the zinc-nickellayer (typically up to 12 to 15% by weight). These complexing agents inturn require a complex wastewater treatment.

DE 2 251 103 A1 describes an acidic zinc electrolyte which is devoid ofboric acid. In addition to zinc sulfate and zinc chloride, theelectrolyte contains sodium succinate, nicotinic acid amide and largeamounts of ammonium chloride. However, the use of ammonium chloride andother ammonium salts is questionable from an environmental perspectivesince neither ammonium nor ammonia should enter the wastewater system.One reason for this is that ammonia is a very good ligand for variousheavy metals and increases the mobility thereof. The use of ammoniumcompounds is not desirable on account of the problematic wastewatertreatment thereof.

U.S. Pat. No. 4,877,497 describes an acidic zinc electrolyte which doesnot contain boric acid. The electrolyte substantially contains zincchloride, ammonium chloride and/or potassium chloride. Moreover,succinic acid, acetic acid, lactic acid, malonic acid, adipic acid,tartaric acid and citric acid or the salts thereof may be contained. ThepH is preferably between 3 and 4. Ammonium chloride requires a complexprocess wastewater treatment, as already described above. The low pHvalues in these electrolytes result in a high solubility of the zincanodes (anode solubility). The high anode solubility means that a steadyincrease in the zinc concentration in the electrolyte is observed whenthe electrolyte is not in operation.

As described above, it is desirable primarily for health reasons and interms of occupational health and safety to reduce the use of boric acidin zinc electrolytes and in particular to avoid it altogether.Similarly, other CMR substances, i.e. substances that are carcinogenic,mutagenic or toxic to reproduction, should be avoided. Ammonium chlorideshould be avoided for environmental reasons owing to the wastewaterproblem. Thus, there is a need for a zinc electrolyte which requiresneither boric acid nor ammonium chloride and which can nevertheless beeconomically viable and which is largely harmless from a health andenvironmental perspective. The electrolyte should facilitate thedeposition of glossy zinc coatings even at high current densities.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a zinc electrolyte forthe electrodeposition of zinc coatings which does not require theaddition of boric acid and ammonium compounds. In particular, the zincelectrolyte should be able to be operated at high current densities andit should use buffer substances that are largely harmless from anenvironmental and health perspective. The electrolyte should, inparticular, facilitate the deposition of glossy zinc coatings having anadvantageous optical impression. Moreover, the electrolyte shouldpreferably also facilitate the electrodeposition of zinc coatings thatcan be annealed, which even after the annealing process have goodmechanical properties and convey a good optical impression. Additionalobjects are to provide a method for producing the zinc electrolyte and amethod for producing a component having a zinc coating which uses theelectrolyte for electrodepositing zinc.

These objects are achieved by the zinc electrolyte, the method for theproduction thereof and the method for producing a component having azinc coating as according to the independent claims.

In particular, the present invention relates to the following:

-   (1) An aqueous electrolyte devoid of boric acid and ammonium for the    electrodeposition of zinc coatings, comprising:-   (a) Zn²⁺ in a concentration of 15 to 70 g/L;-   (b) Cl⁻ in a concentration of 100 to 200 g/L;-   (c) K⁺ and/or Na⁺ in a total concentration of 0.75 to 6.0 mol/L;-   (d) acetate in a concentration of 5.0 to 45 g/L;-   (e) glycine and/or alanine in a total concentration of 0.5 to 30    g/L; and-   (f) water;    -   wherein the electrolyte has a pH of 4.5 to 6.5.-   (2) The electrolyte according to (1), wherein the concentration of    Zn²⁺ is 20 to 60 g/L.-   (3) The electrolyte according to (1) or (2), wherein the    concentration of Zn²⁺ is 25 to 50 g/L, preferably 35 g/L.-   (4) The electrolyte according to any one of (1) to (3), wherein the    concentration of Cl⁻ is 120 to 190 g/L.-   (5) The electrolyte according to any one of (1) to (4), wherein the    concentration of Cl⁻ is 130 to 180 g/L, preferably 160 g/L.-   (6) The electrolyte according to any one of (1) to (5), wherein the    concentration of K⁺ is 0.75 to 6.0 mol/L.-   (7) The electrolyte according to any one of (1) to (6), wherein the    concentration of K⁺ is 2.7 to 4.8 mol/L.-   (8) The electrolyte according to any one of (1) to (7), wherein the    concentration of K⁺ is 3.3 to 4.1 mol/L.-   (9) The electrolyte according to any one of (1) to (8), optionally    containing Na⁺ at 0 to 0.5 mol/L, preferably 0 to 0.2 mol/L.-   (10) The electrolyte according to any one of (1) to (9), wherein    Zn²⁺, K⁺ and Na⁺ account for at least 95% by weight, preferably at    least 98% by weight, most preferably at least 99% by weight or 100%    by weight, of all cations in the electrolyte.-   (11) The electrolyte according to any one of (1) to (10), wherein    Zn²⁺ and K⁺ account for at least 90% by weight, preferably at least    95% by weight, most preferably at least 99% by weight or 100% by    weight, of all cations in the electrolyte.-   (12) The electrolyte according to any one of (1) to (11), wherein    the concentration of acetate is 7.5 to 30 g/L.-   (13) The electrolyte according to any one of (1) to (12), wherein    the concentration of acetate is 10 to 20 g/L, preferably 12 g/L.-   (14) The electrolyte according to any one of (1) to (13), wherein    the total concentration of glycine and/or alanine is in the range of    0.5 to 20 g/L.-   (15) The electrolyte according to any one of (1) to (14), wherein    the total concentration of glycine and/or alanine is in the range of    1.0 to 10 g/L.-   (16) The electrolyte according to any one of (1) to (15), wherein    the total concentration of glycine and/or alanine is in the range of    1.5 to 5 g/L, preferably 2.5 g/L.-   (17) The electrolyte according to any one of (1) to (16), containing    glycine and optionally 0 to 1.5 g/L, preferably 0 to 0.5 g/L, of    alanine.-   (18) The electrolyte according to any one of (1) to (17),    containing (g) nicotinic acid in a concentration of 0.01 to 2.0 g/L.-   (19) The electrolyte according to any one of (1) to (18),    containing (g) nicotinic acid in a concentration of 0.5 to 1.0 g/L.-   (20) The electrolyte according to any one of (1) to (19),    containing (g) nicotinic acid in a concentration of 0.08 to 0.5 g/L,    preferably 0.1 g/L.-   (21) The electrolyte according to any one of (1) to (20),    containing (h) ethoxylated thiodiglycol, with an average of at least    20 structural units derived from ethylene oxide, in a concentration    of 0.3 to 10 g/L.-   (22) The electrolyte according to any one of (1) to (21),    containing (h) ethoxylated thiodiglycol, with an average of at least    20 structural units derived from ethylene oxide, in a concentration    of 0.5 to 5 g/L.-   (23) The electrolyte according to any one of (1) to (22),    containing (h) ethoxylated thiodiglycol, with an average of at least    20 structural units derived from ethylene oxide, in a concentration    of 1.0 to 3.5 g/L, preferably 2.1 g/L.-   (24) The electrolyte according to any one of (1) to (23), wherein    the pH is 4.5 to 6.0, preferably 4.8 to 5.3.-   (25) A method for producing an electrolyte according to any one    of (1) to (24), comprising the steps of:    -   A) forming an aqueous solution of        -   (a′) zinc chloride and/or zinc acetate;        -   (b′) potassium chloride and/or sodium chloride;        -   (c′) at least one of potassium acetate, sodium acetate and            acetic acid;        -   (d′) at least one from the group consisting of glycine, a            salt thereof, alanine and a salt thereof; and        -   (e′) optionally nicotinic acid or a salt thereof; and        -   (f′) optionally ethoxylated thiodiglycol, with an average of            at least 20 structural units derived from ethylene oxide;            and    -   B) optionally setting the pH to 4.5 to 6.5 by adding        hydrochloric acid or by adding potassium hydroxide and/or sodium        hydroxide, which can be added as solids or in the form of an        aqueous solution.-   (26) A method for producing a component having a zinc coating,    comprising the electrodeposition of zinc on a metal component from    an electrolyte according to any one of (1) to (24).-   (27) The method according to (26), wherein the metal component    comprises or consists of iron or an iron alloy.-   (28) The method according to (26) or (27), wherein deposition occurs    at a temperature of 20 to 50° C.-   (29) The method according to any one of (26) to (28), wherein    deposition occurs at a temperature of 25 to 40° C.-   (30) The method according to any one of (26) to (29), wherein during    deposition the current density is from 0.2 to 10 A/dm².-   (31) The method according to any one of (26) to (30), wherein during    deposition the current density is from 0.5 to 8.0 A/dm², preferably    from 1.0 to 8.0 A/dm².-   (32) The method according to any one of (26) to (31), wherein during    deposition the current density is from 0.5 to 6 A/dm², preferably    from 2.0 to 6 A/dm².-   (33) The method according to any one of (26) to (32), wherein after    the electrodeposition the component having a zinc coating is    subjected to a passivation treatment.-   (34) The method according to any one of (26) to (32), wherein after    the electrodeposition the component having a zinc coating is    annealed, optionally before or after a passivation treatment.-   (35) The method according to (34), wherein the temperature during    annealing is 180 to 230° C., preferably 200 to 230° C.-   (36) The method according to (34) or (36), wherein annealing is    carried out for 2 to 24 hours.

DETAILED DESCRIPTION

(1) An aqueous electrolyte devoid of boric acid and ammonium is providedfor the electrodeposition of zinc coatings (herein also referred to inshort as “electrolyte” or “zinc electrolyte”). The electrolytecomprises:

-   (a) Zn²⁺ in a concentration of 15 to 70 g/L;-   (b) Cl⁻ in a concentration of 100 to 200 g/L;-   (c) K⁺ and/or Na⁺ in a total concentration of 0.75 to 6.0 mol/L;-   (d) acetate in a concentration of 5.0 to 45 g/L;-   (e) glycine and/or alanine in a total concentration of 0.5 to 30    g/L; and-   (f) water.

The electrolyte has a pH of 4.5 to 6.5. At this pH, acetate may bepresent in protonated and/or deprotonated form. Similarly, glycineand/or alanine may also be present in protonated and/or deprotonatedform.

Surprisingly, it has been found that with the specific combination ofacetate with glycine and/or alanine in the aforementionedconcentrations, neither the addition of boric acid nor the addition ofammonium is necessary to provide a versatile zinc electrolyte for theelectrodeposition of zinc coatings. Acetate and glycine or alanineserve, inter alia, as buffer substances, for keeping the pH in theelectrolyte constant. Of course, from both a health perspective and anenvironmental perspective, acetate and the amino acids glycine andalanine are completely harmless and can be disposed of via thewastewater system without any problems. Moreover, acetate, glycine andalanine are inexpensive and there is an almost limitless supply thereof,and therefore they constitute a particularly advantageous replacementfor the use of boric acid or related boron compounds as well as ammoniumcompounds such as ammonium chloride.

With the electrolyte according to the invention, zinc coatings can bedeposited over a practicable current density range such as 0.2 to 8A/dm². Particularly at high current densities, i.e. in the range of 6 to8 A/dm², significant improvements can be achieved in the quality of thedeposited zinc coatings in terms of the optical impression, i.e.homogenous gloss and reduced burn marks compared with zinc electrolyteswithout these substances. “Burn marks” are generally to be understood tobe discolored, dark, amorphous or coarsely crystalline, mostly powderyregions, as a result of which the zinc coating may be unusable for manyapplications.

The electrolyte is suitable for coating metal components (substrates)with dense, homogenous zinc coatings that provide good, adhesiveprotection against corrosion, for example on iron or iron alloys.Furthermore, with the zinc electrolyte it was possible to produce zinccoatings that can be annealed. With the buffer substances acetate,glycine and/or alanine, it was possible to reduce hydrogen developmentduring the electrodeposition, in particular at high current densities,as a result of which pH changes in the electrolyte and the precipitationof zinc hydroxide at the cathode were reduced or avoided.

A further advantage resulting from the combination of acetate andglycine is that a versatile process for electrodeposition isfacilitated. For example, zinc salts and conducting salts (potassiumchloride and/or sodium chloride) can be used in wide concentrationranges and the temperature can be varied over a wide range such as 15 to50° C. The electrolyte is also suitable for conventional depositiondevices such as drum or rack devices.

A further advantage of these buffer compounds is that they only weaklybind to most foreign metals such as iron. Iron can therefore easily beprecipitated and removed in the form of iron(III) hydroxide (Fe(OH)₃) inthe presence of these buffer compounds. Furthermore, the buffercompounds do not increase anode solubility (generally zinc anodes) ifthe electrolyte is not in operation. This is advantageous since theconcentrations in the electrolyte that is not in operation largelyremain constant.

(2) The zinc salts naturally serve to provide the metal to be depositedwith zinc. Since the zinc salts are dissolved in the electrolyte, it isoften the case that it can no longer be determined which counterion waspresent in the zinc salt. For this reason, the essential Zn²⁺concentration in the electrolyte is specified herein. In principle, allwater-soluble zinc salts such as zinc sulfate, zinc methane sulfonate,zinc acetate and zinc chloride, for example, can be used. It goeswithout saying that all types of salts, such as anhydrous salts or saltswith water of crystallization, can be used. Preferably, zinc acetateand/or zinc chloride are used, since the anions acetate and chloride areadvantageous for the buffer effect and improving the conductivity of theelectrolyte and do not adversely affect the electrodeposition of zinc.Particularly preferably, zinc chloride is used, on account of its betterconductivity, its good solubility and for reasons of economy.

As already mentioned above, the zinc concentration can be varied over awide range and the electrolyte can therefore be specifically adapted todifferent requirements. The concentration of Zn²⁺ in the electrolyte is15 to 70 g/L. The concentration of Zn²⁺ is, in particular, 20 to 60 g/L,preferably 25 to 50 g/L, more preferably 30 to 40 g/L and mostpreferably 35 g/L. If the zinc concentration is too low, the depositionspeed reduces. When zinc concentrations are too high, solubilityproblems can arise. With the zinc concentrations according to theinvention, the deposition speed, the stability of the zinc electrolyteand the quality of the zinc coatings are all particularly good.

(3) In addition to zinc, which, of course, is required to form the zinccoating, the electrolyte also contains one or more conducting salts.Conducting salts increase the conductivity of the electrolyte, which isimportant in order to be able to coat with zinc at high currentdensities and at a good deposition rate. In particular, potassiumchloride and/or sodium chloride are used as conducting salts.Preferably, potassium chloride is used, since it has a higherconductivity than sodium chloride.

The conducting salt is of course present in the electrolyte in adissolved state, which is why the concentrations of the respective ionsare specified herein. The concentration of Cl⁻ in the electrolyte is 100to 200 g/L. The concentration of Cl⁻ can, in particular, be 120 to 190g/L, preferably 130 to 180 g/L and more preferably 160 g/L. With thesechloride concentrations and the corresponding conducting saltconcentrations, the electrolyte can be operated well.

(4) The cations K⁺ and/or Na⁺ can enter the electrolyte from variousreagents. On the one hand, K⁺ and/or Na⁺ originate from the conductingsalt described above. Furthermore, these ions can also be introducedinto the electrolyte as a component of other salts. For instance,acetate, glycine and/or alanine and other weak acids, also in the formof the corresponding potassium salt and/or sodium salt, can be used forforming the electrolyte.

Potassium and sodium are, of course, harmless both from a healthperspective and from an environmental perspective and do not interferewith the operation of the electrolyte. The electrolyte may containfurther cations in addition to Zn²⁺ and K⁺ and/or Na⁺, but it does nothave to. Preferably, Zn²⁺, K⁺ and Na⁺ account for at least 95% byweight, more preferably at least 98% by weight, most preferably at least99% by weight or 100% by weight, of all cations in the electrolyte. Dueto the advantages of potassium chloride as described above, theelectrolyte preferably contains Zn²⁺ and Kt Zn²⁺ and K⁺ can account forat least 90% by weight, more preferably at least 95% by weight, mostpreferably at least 99% by weight or 100% by weight, of all cations inthe electrolyte.

The total concentration of K⁺ and/or Na⁺ in the electrolyte is in therange of 0.75 to 6.0 mol/L. Due to the difference in mass between K⁺ andNa⁺, the molar concentration is specified herein. Preferably, theelectrolyte contains K⁺ and optionally Na⁺, wherein the concentration ofK⁺ is 0.75 to 6.0 mol/L, in particular 2.7 to 4.8 mol/L, preferably 3.3to 4.1 mol/L. Preferably, the concentration of Na⁺ is 0.5 mol/L or lessand more preferably it is 0.2 mol/L or less, which includes 0 mol/L(i.e. 0 to 0.5 mol/L and 0 to 0.2 mol/L respectively).

(5) As already mentioned above, the electrolyte contains acetate as abuffer substance. With the pH in the electrolyte of 4.5 to 6.5, theacetate (AcO⁻) is naturally in equilibrium with the conjugate acid(AcOH=acetic acid). AcO⁻ and AcOH form a conjugate acid base pair, orthe deprotonated and protonated form of the acetate. With low pH values,the proportion of conjugate acids (protonated form) is higher than withhigher pH values. At least one of potassium acetate, sodium acetate andacetic acid, preferably potassium acetate, can be used as the acetatereagent, i.e. as the source for the acetate in the electrolyte.

The specification of the concentration of acetate refers to the mass ofthe acetate ion (AcO⁻), i.e. the mass of the deprotonated form,irrespective of the size of the equilibrium proportion of the conjugateacid (AcOH), i.e. the protonated form. It can be calculated from theadded amount of the acetate reagent. In the electrolyte, theconcentration of acetate is 5.0 to 45 g/L. In particular, it can be 7.5to 30 g/L, preferably 10 to 20 g/L, most preferably 12 g/L.

As the buffer substance, the acetate serves firstly to keep the pH ofthe electrolyte constant. The acetate also helps to create glossy zinccoatings that can be annealed. Acetate proved to be essential for theoperation of the electrolyte, particularly at high current densities. Ifthe acetate concentration (which includes the conjugate acid, asdescribed above) is too low, the desired effects are only achieved to anunsatisfactory degree. With higher concentrations, solubility problemscan arise. The cost-effectiveness of the electrolyte and the quality ofthe zinc coatings are best in the preferred ranges.

(6) As already stated above, in addition to acetate, the electrolytealso contains glycine and/or alanine, which also serve as buffersubstances. Glycine and/or a water-soluble salt thereof can be used asthe source of the glycine. Alanine and/or a water-soluble salt thereofcan be used as the source of the alanine. Preferably, at least one ofglycine, potassium salt of glycine (potassium glycinate), sodium salt ofglycine (sodium glycinate), alanine, potassium salt of alanine(potassium alaninate) and sodium salt of alanine (sodium alaninate) isused. Particularly preferably, just glycine and/or alanine is used. Mostpreferably, glycine is used.

Glycine has two pK_(s)-values, such that depending on the pH of 4.5 to6.5, glycine can exist in equilibrium in several species in theelectrolyte. Glycine can in principle be fully protonated(H₃N—CH₂—CO₂H⁺, protonated form), neutral to the outside (H₂N—CH₂—CO₂Hor H₃N⁺—CH₂—CO₂ ⁻) and fully deprotonated (H₂N—CH₂—CO₂ ⁻, deprotonatedform) in the electrolyte. The species each form conjugate acid basepairs. The same applies to alanine, which can also be fully protonated(H₃N—CHCH₃—CO₂H⁺, protonated form), neutral to the outside(H₂N—CHCH₃—CO₂H or H₃N⁺—CHCH₃—CO₂ ⁻) and fully deprotonated(H₂N—CHCH₃—CO₂ ⁻, deprotonated form) in the electrolyte.

The specification that acetate, glycine and/or alanine can also bepresent in protonated and/or deprotonated form shows that acetate,glycine and/or alanine can be present in all protonated, deprotonated oroutwardly neutral species that occur at the pH values of 4.5 to 6.5. Atthe corresponding pH, the species form in equilibrium in the electrolyteon their own, as is typical of weak acids and as is known to personsskilled in the art.

The concentration of the glycine and/or alanine is calculated inrelation to the mass of glycine (H₂N—CH₂—CO₂H) and/or alanine(H₂N—CHCH₃—CO₂H) as such, irrespective of which of the species is formedor is present in equilibrium. In the electrolyte, the totalconcentration of glycine and/or alanine is 0.5 to 30 g/L. Theconcentration of glycine and/or alanine can, in particular, be 0.5 to 20g/L, preferably 1.0 to 10 g/L, more preferably 1.5 to 5 g/L and mostpreferably 2.5 g/L. If the electrolyte contains too little glycineand/or alanine, the desired effects are only achieved to anunsatisfactory degree. If the amounts are too large, solubility problemscan arise.

Preferably, the electrolyte contains glycine and optionally alanine. Inthis embodiment, the electrolyte preferably contains glycine in aconcentration of 0.5 to 20 g/L, more preferably 1.0 to 10 g/L, even morepreferably 1.5 to 5 g/L and most preferably 2.5 g/L, and alanine in aconcentration of less than 1.5 g/L and more preferably 0.5 g/L or less,which includes 0 g/L (i.e. 0 to 1.5 g/L and 0 to 0.5 g/L respectively).

As already mentioned above, the pH of the electrolyte is 4.5 to 6.5.Preferably, it can be 4.5 to 6.0 and more preferably 4.8 to 5.3. Atthese pH values, good deposition rates and low anode solubilities areobserved. The pH of the electrolyte can result from the composition ofthe components alone. It is also possible to set the pH, as will bedescribed below.

According to a further embodiment, the electrolyte also contains one ormore further buffer substances. The further buffer substance is, inparticular, at least one buffer substance selected from the groupconsisting of succinic acid, adipic acid, malic acid,2-(N-morpholino)ethanesulfonic acid, tris(hydroxymethyl)aminomethane,triethanolamine, taurine, β-alanine, glutamic acid, glycylglycine,threonine, bicine, tricine, ascorbic acid, citric acid and saltsthereof. In particular, the salts are potassium and/or sodium salts.

The further buffer substance is therefore optional and can be used in aconcentration of 0.1 to 40 g/L.

(7) It goes without saying that the various embodiments of thecomponents present in the electrolyte can be combined as desired.According to a preferred variant, the electrolyte contains:

-   (a) Zn²⁺ in a concentration of 20 to 60 g/L;-   (b) Cl⁻ in a concentration of 120 to 190 g/L;-   (c) K⁺ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.5 mol/L    Na⁺;-   (d) acetate in a concentration of 7.5 to 30 g/L; and-   (e) glycine and/or alanine in a total concentration of 0.5 to 20    g/L.

According to a preferred development of this variant, the electrolytecontains:

-   (a) Zn²⁺ in a concentration of 20 to 60 g/L;-   (b) Cl⁻ in a concentration of 120 to 190 g/L;-   (c) K⁺ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.5 mol/L    Na⁺;-   (d) acetate in a concentration of 10 to 20 g/L; and-   (e) glycine and/or alanine in a concentration of 1.0 to 10 g/L.

According to a further preferred development of this variant, theelectrolyte contains:

-   (a) Zn²⁺ in a concentration of 20 to 60 g/L;-   (b) Cl⁻ in a concentration of 120 to 190 g/L;-   (c) K⁺ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.2 mol/L    Na⁺;-   (d) acetate in a concentration of 10 to 20 g/L; and-   (g) glycine in a concentration of 1.5 to 5 g/L and 0 to 0.5 g/L    alanine.

(8) According to a further embodiment, the electrolyte additionallycontains (g) nicotinic acid and/or (h) ethoxylated thiodiglycol.Particularly preferably, the electrolyte contains nicotinic acid andethoxylated thiodiglycol.

Nicotinic acid as such or water-soluble salts thereof, in particular thepotassium and/or sodium salt thereof, can be used as the source of thenicotinic acid. Preferably, just nicotinic acid is used.

Since nicotinic acid is a weak acid, at the pH of 4.5 to 6.5 it canexist in the electrolyte in an equilibrium of protonated anddeprotonated forms, i.e. as a conjugate acid base pair. Thus,analogously to acetate and glycine and/or alanine, nicotinic acid can bepresent in the form of the conjugate base and/or the conjugate acid.

The concentration is calculated in relation to the mass of the nicotinicacid (C₆H₅NO₂) as such. The concentration of nicotinic acid in theelectrolyte is, in particular, 0.01 to 2.0 g/L, preferably 0.05 to 1.0g/L, more preferably 0.08 to 0.5 g/L, most preferably 0.1 g/L.

Quite surprisingly, it has been found that by adding small amounts ofnicotinic acid, the quality of the zinc coatings is significantlyimproved further still, even at high current densities such as 6 to 8A/dm². In addition to the optical impression (gloss) of the depositedzinc coatings, the ability thereof to anneal, in particular, is alsofurther improved. Even at high current densities, it is possible toproduce zinc coatings which can be annealed and which after annealinghave no or very few defects such as bubbles and create a particularlyadvantageous optical impression, i.e. a homogenous gloss withoutcloudiness or fog. If the concentration of nicotinic acid is too high,the formation of fog or cloudiness in the zinc coating can increaseagain.

(9) Ethoxylated thiodiglycol is an oligomeric or polymeric compoundobtained from the addition reaction of thiodiglycol(HO—CH₂—CH₂—S—CH₂—CH₂—OH) with ethylene oxide (oxirane, C₂H₄O). Asuitable ethoxylated thiodiglycol has an average of at least 20structural units derived from ethylene oxide. Preferably, it has anaverage of 20 to 100 structural units derived from ethylene oxide. Withfewer structural units derived from ethylene oxide, the advantageouseffects could not be observed to the same degree.

Ethoxylated thiodiglycol can be added to the electrolyte in the form ofan aqueous solution. Ethoxylated thiodiglycol is commercially availableas a 70% aqueous solution, for example, such as CHE ED 7127 70% (fromthe company Erbslöh) or Aduxol TDG-027 70% (from Schärer & Schläpfer).

The concentration of ethoxylated thiodiglycol with an average of atleast 20 structural units derived from ethylene oxide in the electrolytecan be 0.3 to 10 g/L, preferably 0.5 to 5 g/L, more preferably 1.0 to3.5 g/L and most preferably 2.1 g/L.

Quite surprisingly, it has been found that also with the ethoxylatedthiodiglycol described above the quality of the zinc coatings issignificantly improved further still, even at high current densitiessuch as 6 to 8 A/dm². In addition to the optical impression (gloss) ofthe deposited zinc coatings, the ability thereof to anneal, inparticular, is also further improved. Even at high current densities, itis possible to produce zinc coatings which can be annealed and whichafter annealing have no or very few defects such as bubbles and have anadvantageous optical impression. With concentrations that are too high,the optical impression of the zinc coatings worsens again.

(10) According to a further variant of the electrolyte, it contains

-   (g) nicotinic acid in a concentration of 0.05 to 1.0 g/L, and/or-   (h) ethoxylated thiodiglycol, with an average of at least 20    structural units derived from ethylene oxide, in a concentration of    0.5 to 5 g/L.

The advantageous effects of nicotinic acid and ethoxylated thiodiglycolare very similar, as already stated above, even though the substancesare structured quite differently. It has also surprisingly been foundthat by using a combination of nicotinic acid and ethoxylatedthiodiglycol, the advantageous properties of the zinc coatings can beimproved further still. Preferably, the electrolyte therefore containsnicotinic acid and ethoxylated thiodiglycol, in particular in theconcentrations specified above.

In addition to the aforementioned components, the electrolyte can alsocontain one or more common additives for zinc electrolytes inappropriate quantities, as is already known to persons skilled in theart. The electrolyte can contain, for example, at least one additivefrom the group consisting of surfactants (e.g. alkoxylated (3-naphthol),basic brighteners (e.g. sodium benzoate), solubilizers or hydrotropes(e.g. sodium cumene sulfonate) and brighteners (e.g. benzalacetoneand/or o-chlorobenzaldehyde). Surfactants serve to reduce the surfacetension of the electrolyte and ensure a good wetting of the surface tobe coated. Solubilizers and hydrotropes ensure a sufficient solubilityof organic substances. Basic brighteners and brighteners are used incombination to further control the degree of gloss and brilliance of thezinc coating.

Suitable additives are commercially available. For example, SLOTANIT BSF1668 (an additive for zinc electrolytes which contains surfactants andbasic brighteners) and SLOTANIT BSF 1662 (an additive for zincelectrolytes which contains solubilizers and brighteners) can beobtained from Schlötter. These additives can be used in the usualquantities according to the corresponding instructions for use of themanufacturer.

(11) As a further aspect of the invention, a method for producing theelectrolyte according to any one of the embodiments described above isprovided. The method comprises the steps of

-   A) forming an aqueous solution of    -   (a′) zinc chloride and/or zinc acetate;    -   (b′) potassium chloride and/or sodium chloride;    -   (c′) at least one of potassium acetate, sodium acetate and        acetic acid;    -   (d′) at least one from the group consisting of glycine, a salt        thereof, alanine and a salt thereof; and    -   (e′) optionally nicotinic acid or a salt thereof; and    -   (f′) optionally ethoxylated thiodiglycol, with an average of at        least 20 structural units derived from ethylene oxide; and-   B) optionally setting the pH to 4.5 to 6.5 by adding hydrochloric    acid or by adding potassium hydroxide and/or sodium hydroxide, which    can be added as solids or in the form of an aqueous solution.

Thus, in step A), an aqueous solution of the components is formed. Thecomponents can be added in the specified order or in any other order.Further buffer substances and additives, as described above, can also becontained in the aqueous solution. In particular, (g′) commonsurfactants, basic brighteners, solubilizers and brighteners can beadded to the aqueous solution, such as the additives SLOTANIT BSF 1668and SLOTANIT BSF 1662 from Schlötter. With regard to the components andthe respective embodiments of the electrolyte, reference is made to thestatements made above.

In order to accelerate step A), in particular steps (a′) to (c′), themixture can be mixed, in particular stirred. Furthermore, heating, forexample to 60° C., can also be used for acceleration. Once the aqueoussolution has formed, it can be cooled down again to a lower temperature.

The pH of the electrolyte of 4.5 to 6.5 can result from the compositionof the components alone. It is also possible to set the pH in anoptional step B). This can be done by adding hydrochloric acid, toreduce the pH. The hydrochloric acid can, for example, be diluted (1 N)or be made more concentrated. Concentrated hydrochloric acid (ca. 12 N)can also be used. 5 or 6 N (semi-concentrated) hydrochloric acid isparticularly suitable since it can be handled safely and does not leadto the electrolyte being significantly diluted.

By adding potassium hydroxide and/or sodium hydroxide, the pH can bereduced. These bases can be added as solids or in the form of an aqueoussolution. For example, 50% potassium hydroxide solution (KOH) can beused.

Hydrochloric acid, potassium hydroxide and sodium hydroxide areparticularly suitable since they do not introduce additional ions intothe electrolyte.

(12) As a further aspect of the invention, a method for producing acomponent having a zinc coating is provided. The method comprises theelectrodeposition of zinc on a metal component from an electrolyteaccording to any one of the embodiments described above.

(13) In principle, any metal substrate that is suitable for theelectrodeposition of zinc can be used as the component. Preferably, thecomponent comprises iron or an iron alloy or it consists wholly thereof.Accessory parts for the automotive industry, for example, can beprovided with zinc coatings. The method can also be particularlyeffectively used, for example, for the electrodeposition of zinccoatings on components made of cast materials such as wrought iron orgray cast iron.

In general, the component is pre-treated prior to the electrodeposition,in order to clean it, in particular to degrease it. Appropriate measuresare known to persons skilled in the art and suitable reagents arecommercially available. For example, the component, such as a steelsheet, can first be 1) degreased in a hot cleaning solution, 2) pickledin acid (e.g. semi-concentrated hydrochloric acid), 3) electrolyticallydegreased and then 4) pickled with diluted hydrochloric acid. After eachof steps 1 to 4), it is rinsed with water.

(14) Customary conditions can be used for the electrodeposition. Asstated above, with the electrolyte according to the invention it ispossible to vary the temperature over a wide range. The temperature forthe electrodeposition can, in particular, be 20 to 50° C., preferably 25to 40° C.

Moreover, practicable current densities, such as 0.2 to 10 A/dm², inparticular 0.5 to 8.0 A/dm², preferably 1.0 to 8.0 A/dm² or 0.5 to 6A/dm², more preferably 2.0 to 6 A/dm², can be applied. This allows themethod to be operated with good economic efficiency.

If desired or required, the pH can be adjusted during the procedure. Theprocedure can be analogous to step B) described above. Iron canoptionally be precipitated and removed as iron(III) hydroxide.

All conventional devices, such as drum devices or rack devices, aresuitable for the procedure. A plurality of cathodes and/or anodes can beused. Normally, the electrolyte is mixed during the electrodeposition.For this, one or more of stirring devices, circulation pumps (also incombination with Venturi nozzles) or air injection systems can be used,for example.

For the electrodeposition of zinc, the metal component is connected asthe cathode and divalent zinc ions are reduced to metallic zinc on thesurface thereof, as a result of which the zinc coating is formed.Normally, zinc is used as the anode.

(15) According to a further embodiment of the method, after theelectrodeposition the component having a zinc coating is subjected to apassivation treatment. For this, the component having a zinc coating istreated with an appropriate passivation agent, as is known to personsskilled in the art. For this, the coated components can, for example, betreated with a thin-film or thick-film passivation. Such passivationprocesses typically contain chromium(III) and cobalt ions as corrosioninhibitors, as well as further film-forming substances (such asfluorides, sulfates and nitrates). One commercially availablepassivation solution for zinc coatings is, for example, the thin-filmpassivation SLOTOPAS Z 20 Blau, or the passivation concentrate SLOTOPASZ 21 Blau, which are available from Schlötter. The correspondinginstructions for use of the manufacturer can be followed.

The passivation treatment takes place after the electrodeposition.Normally, the components having a zinc coating are firstly rinsed withwater. If the components having a zinc coating are annealed, thepassivation treatment can in principle be carried out before or afterannealing.

(16) According to a further embodiment of the method, after theelectrodeposition the component having a zinc coating is annealed.Annealing drives out the hydrogen absorbed during the electroplatingprocess, in order to counteract the risk of the component subsequentlybecoming embrittled by the hydrogen. Particularly in the case ofhigh-strength steel components, there is an increased risk ofhydrogen-induced brittle fracture due to the material.

For annealing, it is common to heat to a temperature of 180 to 230° C.,preferably 200 to 230° C. Normally, annealing is carried out over a longperiod of time, for example 2 to 24 hours.

Prior to annealing, the component is usually rinsed with water anddried.

In principle, the annealing is optional and can take place irrespectiveof whether or not a passivation treatment is carried out. If apassivation treatment is carried out, this can happen both before andafter the annealing.

To clarify the invention, reference is made to the drawings and to theexamples below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic test set-up for the electrodeposition of zincon an angular steel sheet.

FIG. 2 shows a schematic test set-up for the electrodeposition of zincon a straight steel sheet.

EXAMPLES Production of Electrolytes E1 to E6 Procedure:

The electrolytes were prepared at room temperature by adding deionizedwater to the other components and stirring. The dissolution of thesolids was accelerated by heating them to 60° C. Once the solution hadformed, it was cooled to 25° C.

Where necessary, the pH was set to 5.2 by adding 5 N hydrochloric acidor 50% potassium hydroxide solution (756 g/L KOH).

Reagents Used:

Technical grade potassium chloride was used.

Zinc chloride HP from Schlötter was used as the zinc chloride.

The additives SLOTANIT BSF 1668 (basic additive) and SLOTANIT BSF 1662(brightener additive), which are both available from Schlötter, wereused.

“Purest” quality potassium acetate was used.

“Purest” quality glycine was used.

Technical grade nicotinic acid was used.

A 70% solution, namely CHE ED 7127 70%, which is available from thecompany Erbsloh, or Aduxol TDG-027 70%, which is available from Schärer& Schläpfer, was used as the ethoxylated thiodiglycol.

Components for Electrolyte E1 (Base Electrolyte):

73 g/L ZnCl₂ (corresponds to 35 g/L Zn²⁺ and 38 g/L Cl⁻) 260 g/L KCl(corresponds to 136 g/L K⁺ and 124 g/L Cl⁻) 35.0 ml/L SLOTANIT BSF 16680.3 ml/L SLOTANIT BSF 1662

Components for Electrolyte E2:

73 g/L ZnCl₂ 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANITBSF 1662 20 g/L Potassium acetate (corresponds to 8 g/L K⁺ and 12 g/LAcO⁻)

Components for Electrolyte E3:

73 g/L ZnCl₂ 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANITBSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine

Components for Electrolyte E4:

73 g/L ZnCl₂ 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANITBSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 2.11 g/L CHE ED 712770% or Aduxol TDG-027 70%

Components for Electrolyte E5:

73 g/L ZnCl₂ 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANITBSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 0.113 g/L Nicotinicacid

Components for Electrolyte E6:

73 g/L ZnCl₂ 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANITBSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 2.11 g/L CHE ED 712770% or Aduxol TDG-027 70% 0.113 g/L Nicotinic acid

Pre-Treatment of the Steel Sheets

-   1) Decoction degreasing with SLOTOCLEAN 160 (from Schlötter) for 15    minutes at 65° C. and subsequent rinsing with water.-   2) Hydrochloric acid pickling with 19% hydrochloric acid and 40 mL/L    pickling degreaser SLOTOCLEAN BEF 30 (from Schlötter) for 7 minutes    at 25° C. and subsequent rinsing with water.-   3) Electrolytic degreasing by means of anodic degreasing with    SLOTOCLEAN DCG (from Schlötter) at a cathodic current density of 3    to 6 A/dm² for 2 minutes at 25° C. and subsequent rinsing with    water.-   4) Pickling with diluted hydrochloric acid (1:10 dilution of    concentrated hydrochloric acid) for 1 minute at 25° C. and    subsequent rinsing with water.

Electrodeposition Deposition Parameters in the Electroplated ZincElectrolyte:

Electrolyte volumes: 3.0 L.

pH of the electrolyte: 5.2.

Electrolyte temperature: 25° C.

Stirring speed set on the magnetic stirrer: 300 rpm.

General Procedure:

The test set-up for electrodeposition with an angular sheet of steel isillustrated in FIG. 1. The test set-up for electrodeposition with astraight sheet of steel is illustrated in FIG. 2.

The electrolyte (1) is present in a beaker (10) and was kept permanentlyin motion by means of a magnetic stirrer (15) and a magnetic stirrer bar(length: 40 mm, diameter: 8 mm) (16). A Schott Duran beaker (3.0 L, lowform) was used as the beaker. The temperature was controlled and heldconstant by means of a contact thermometer (17) which is connected tothe heating relay of the magnetic stirrer. In the Examples, a magneticstirrer with a heatable plate, the IKA RET basic model from the companyIKA, was used. A stabilizer (rectifier) (20) from the company GossenMetrawatt, the SLP 240-40 model, served as the power source. A currentstrength of 3.0 A and a voltage of 2.5 V are illustrated by way ofexample.

Two anodes (2, 2′) were used for the electrodeposition. High-grade zincanodes (99.99% Zn) in accordance with DIN EN 1179 were used as the anodematerial (length: 10 cm, width: 5 cm, thickness: 1 cm). The anodes wereimmersed 9 cm deep in the electrolyte.

Before the cathode sheets were introduced, they were subjected to thepre-treatment for steel sheets described above.

The cathode sheet (3) was arranged in the middle of the two anodes inthe beaker. The distance to the anodes was 6 cm both from the front sideand the rear side of the cathode sheet. The cathode sheet is immersed sodeeply in the electrolyte that the total immersed area (front and rearsides) is one square decimeter.

Where necessary, the pH was set to 5.2 by adding 5 N hydrochloric acidor 50% potassium hydroxide solution (756 g/L KOH) before the start ofthe deposition.

In each case, the electrodeposition was performed from the respectiveelectrolyte at the current density specified in the Examples.

Passivation:

After the electrodeposition of zinc, the coated sheets were thoroughlyrinsed with water and subsequently brightened in diluted hydrochloricacid (15 mL 25% HCl in 1 L water). Following another rinse with water,passivation was carried out in a thin-film passivation SLOTOPAS Z 20blau (prepared with the passivation concentrate SLOTOPAS Z 21 BLAU fromSchlötter, 35 mL/L) at 25° C., pH=1.9, and 60 seconds immersion time.The coated sheets were subsequently rinsed with water and dried at 80°C. for 15 minutes in a convection oven. Prior to assessment or furthertreatment of the sheets, they were cooled to room temperature.

Test A): Burn Marks on the Angular Sheet Examples 1 to 4, ComparativeExamples 1 and 2

Angular sheets having type 2 geometries according to DIN 50957-2 wereused. The material of the angular sheets was cold-rolled steel accordingto DIN EN 10139/10140 (quality: DC03 LC MA RL). The angular sheets werepre-treated in accordance with the procedure described above and thencoated. The electrolytes E1 to E6 as specified in Table 1 were used forComparative Examples 1 and 2 (CE1 and CE2) as well as for Examples 1 to4. The current density was varied and was 0.25, 1.0, 2.0, 4.0, 6.0 or8.0 A/dm². The deposited layer thicknesses were 10 μm. The sheets werethen passivated as described above.

The appearance of the deposited zinc coating in the edge region of theangular sheets and the number of burn marks (discolored, dark, amorphousor coarsely crystalline, mostly powdery regions in the coating) whichcan be caused by increased local current densities were assessed by avisual inspection.

Key:

Δ no burn marks∘ mild burn marks● moderate burn marks▴ major burn marks

The results are summarized in Table 1.

TABLE 1 Burn Marks on the Angular Sheet Current Density in A/dm²Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE1 E1 Δ Δ Δ ◯ ● ▴ CE2 E2 ΔΔ Δ ◯ ◯ ● 1 E3 Δ Δ Δ ◯ ◯ ◯ 2 E4 Δ Δ Δ Δ Δ ◯ 3 E5 Δ Δ Δ Δ Δ ◯ 4 E6 Δ Δ ΔΔ Δ Δ

At low and average current densities of up to 2 A/dm², a usable zinccoating was deposited from all of the electrolytes E1 to E6. At highcurrent densities of 4 or 8 A/dm², considerable differences between theindividual electrolytes were shown.

In Comparative Example 1 (CE1), electrolyte E1, which is devoid of boricacid and ammonium, resulted without the buffer substances acetate andglycine in unfit zinc coatings which have massive burn marks on the edgeregions thereof. The addition of acetate allowed initial improvements tobe achieved in Comparative Example 2 (CE2) with electrolyte E2, but toomany burn marks were still observed at 8 A/dm².

In contrast, high-quality zinc coatings were obtained in Example 1 withelectrolyte E3 even at high current densities, which coatings exhibitedonly minor burn marks even at 8 A/dm². Thus, electrolyte E3 according tothe invention constitutes a veritable boric acid-free and ammonium-freesubstitute for conventional electrolytes containing boric acid.

In Examples 2 to 4, a further considerable improvement was then achievedwith the additives ethoxylated thiodiglycol and nicotinic acid.

Test B): Annealability Examples 5 to 8, Comparative Examples 3 and 4

Straight steel sheets with the dimensions 0.3 mm thickness, 50 mm widthand 130 mm length were used. The material of the straight steel sheetswas cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LCMA RL). The steel sheets were pre-treated in accordance with theprocedure described above and then coated. The electrolytes E1 to E6were used for Comparative Examples 3 and 4 (CE3 and CE4) as well as forExamples 5 to 8 (see Tables 2 and 3). The current density was varied andwas 0.25, 1.0, 2.0, 4.0, 6.0 or 8.0 A/dm². The deposited layerthicknesses were 10 μm.

After the electrodeposition of zinc, the sheets were then passivated, asdescribed above, stored for 48 hours at room temperature andsubsequently annealed for 24 hours at 210° C.

Then, the bubble formation caused by annealing was assessed at roomtemperature.

Key:

Δ no bubble formation∘ mild bubble formation● moderate bubble formation▴ major bubble formation

The results are summarized in Table 2.

TABLE 2 Bubble Formation After Annealing Current Density in A/dm²Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE3 E1 Δ Δ Δ ◯ ▴ ▴ CE4 E2 ΔΔ Δ ◯ ▴ ▴ 5 E3 Δ Δ Δ ● ▴ ▴ 6 E4 Δ Δ Δ Δ ◯ ▴ 7 E5 Δ Δ Δ Δ ◯ ◯ 8 E6 Δ Δ ΔΔ Δ Δ

In Example 5, zinc coatings with very good annealability which did notexhibit any bubble formation at all were produced with electrolyte E3,which contains potassium acetate and glycine, at conventional currentdensities between 0.25 and 2 A/dm².

A further improvement in the annealability of zinc coatings deposited athigher current densities was observed for electrolytes E4 and E5 inExamples 6 and 7, which contained ethoxylated thiodiglycol (E4) andnicotinic acid (E5) respectively. By combining both additives inelectrolyte E6 a completely bubble-free annealed zinc coating couldstill be achieved even at a current density of 8 A/dm² (see Example 8).

Moreover, the optical impression (gloss) was assessed at roomtemperature.

Key:

Δ no milky appearance∘ slight milky appearance● moderate milky appearance▴ strong milky appearance

The results are summarized in Table 3.

TABLE 3 Gloss After Annealing Current Density in A/dm²Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE3 E1 ▴ ▴ ▴ ▴ ▴ ▴ CE4 E2 ▴▴ ▴ ▴ ▴ ▴ 5 E3 ▴ ● ● ● ● ● 6 E4 Δ Δ Δ ◯ ◯ ◯ 7 E5 Δ Δ Δ Δ Δ Δ 8 E6 Δ Δ ΔΔ Δ Δ

In Comparative Examples 3 and 4, electrolytes 1 and 2, which are devoidof boric acid and ammonium, resulted in a very milky appearance withnumerous streaks. With electrolyte E3, which contains acetate andglycine, the gloss was improved and the milky appearance was slightlyreduced.

By adding ethoxylated thiodiglycol to electrolyte E4 and in particularby adding nicotinic acid to electrolytes 5 and 6, the gloss of theannealed zinc coatings was improved further still (see Examples 6 to 8).Regardless of the current density used, the zinc coatings only had aslight milky appearance, or no milkiness, cloudiness or streaks at all.

Test C): Thermal Shock Test Examples 9 to 12, Comparative Examples 5 and6

Straight steel sheets with the dimensions 0.3 mm thickness, 50 mm widthand 130 mm length were used. The material of the straight steel sheetswas cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LCMA RL). These were pre-treated in accordance with the proceduredescribed above and then coated at a current density of 3.0 A/dm². Theelectrolytes E1 to E6 were used for Comparative Examples 5 and 6 (CE5and CE6) as well as for Examples 9 to 12 (see Table 4). The depositedlayer thicknesses were 10 μm.

After the electrodeposition of zinc, the sheets were passivated, asdescribed above, stored for 48 hours at room temperature andsubsequently stored for 30 minutes at 220° C. and immediately placed infiltered tap water annealed to 20° C.

First, the tap water was examined and assessed for any flaking.

After the thermal shock test the sheets were dried for 15 minutes at 80°C. in a convection oven and after cooling to room temperature theadhesiveness was visually inspected again. For this, 4 cm long, 19 mmwide adhesive strips (Tesafilm® Crystal Clear from Tesa SE) were stuckon the sheets and removed again after 60 seconds. The zinc coatings werethen inspected for damage.

The two adhesion tests were deemed to have been passed if no flakes orother particles, chips, bubbles or damage were observed.

Key:

Δ Passed (no flaking and good adhesion)

The results are summarized in Table 4.

TABLE 4 Adhesion After Thermal Shock Test Example/ Current DensityElectrolyte 3.0 A/dm² CE5 E1 Δ CE6 E2 Δ  9 E3 Δ 10 E4 Δ 11 E5 Δ 12 E6 Δ

No flaking was observed with any of the electrolytes, and a goodadhesion of the zinc coating on the steel sheet was exhibited. Thus, itis possible to produce adhesion-resistant zinc coatings with theelectrolyte devoid of boric acid and ammonium.

1. An aqueous electrolyte devoid of boric acid and ammonium for theelectrodeposition of zinc coatings, comprising: (a) Zn²⁺ in aconcentration of 15 to 70 g/L; (b) Cl⁻ in a concentration of 100 to 200g/L; (c) K⁺ and/or Na⁺ in a total concentration of 0.75 to 6.0 mol/L;(d) acetate in a concentration of 5.0 to 45 g/L; (e) glycine and/oralanine in a total concentration of 0.5 to 30 g/L; and (f) water;wherein the electrolyte has a pH of 4.5 to 6.5.
 2. The electrolyteaccording to claim 1, wherein the concentration of Zn²⁺ in theelectrolyte is 20 to 60 g/L, preferably 25 to 50 g/L, more preferably 30to 40 g/L and most preferably 35 g/L.
 3. The electrolyte according toclaim 1, wherein the concentration of Cl⁻ in the electrolyte is 120 to190 g/L, preferably 130 to 180 g/L and more preferably 160 g/L.
 4. Theelectrolyte according to claim 1, wherein the electrolyte contains K⁺and the concentration of K⁺ is 0.75 to 6.0 mol/L, preferably 2.7 to 4.8mol/L and more preferably 3.3 to 4.1 mol/L.
 5. The electrolyte accordingto claim 1, wherein the concentration of acetate in the electrolyte is7.5 to 30 g/L, preferably 10 to 20 g/L, most preferably 12 g/L.
 6. Theelectrolyte according to claim 1, wherein the total concentration ofglycine and/or alanine in the electrolyte is in the range of 0.5 to 20g/L, preferably 1.0 to 10 g/L, more preferably 1.5 to 5 g/L and mostpreferably 2.5 g/L.
 7. The electrolyte according to claim 1, wherein theelectrolyte contains (a) Zn²⁺ in a concentration of 20 to 60 g/L; (b)Cl⁻ in a concentration of 120 to 190 g/L; (c) K⁺ in a concentration of2.7 to 4.8 mol/L and 0 to 0.5 mol/L Nat; (d) acetate in a concentrationof 7.5 to 30 g/L; and (e) glycine and/or alanine in a totalconcentration of 0.5 to 20 g/L.
 8. The electrolyte according to claim 1,wherein the electrolyte additionally contains (g) nicotinic acid in aconcentration of 0.01 to 2.0 g/L, preferably 0.05 to 1.0 g/L, morepreferably 0.08 to 0.5 g/L, most preferably 0.1 g/L.
 9. The electrolyteaccording to claim 1, wherein the electrolyte additionally contains (h)ethoxylated thiodiglycol, with an average of at least 20 structuralunits derived from ethylene oxide, in a concentration of 0.3 to 10 g/L,preferably 0.5 to 5 g/L, more preferably 1.0 to 3.5 g/L and mostpreferably 2.1 g/L.
 10. The electrolyte according to claim 1, whereinthe electrolyte contains (g) nicotinic acid in a concentration of 0.05to 1.0 g/L, and/or (h) ethoxylated thiodiglycol, with an average of atleast 20 structural units derived from ethylene oxide, in aconcentration of 0.5 to 5 g/L.
 11. A method for producing an aqueouselectrolyte according to claim 1, comprising the steps of: A) forming anaqueous solution of (a′) zinc chloride and/or zinc acetate; (b′)potassium chloride and/or sodium chloride; (c′) at least one ofpotassium acetate, sodium acetate and acetic acid; (d′) at least onefrom the group consisting of glycine, a salt thereof, alanine and a saltthereof; and (e′) optionally nicotinic acid or a salt thereof; and (f′)optionally ethoxylated thiodiglycol, with an average of at least 20structural units derived from ethylene oxide; and B) optionally settingthe pH to 4.5 to 6.5 by adding hydrochloric acid or by adding potassiumhydroxide and/or sodium hydroxide, which can be added as solids or inthe form of an aqueous solution.
 12. A method for producing a componenthaving a zinc coating, comprising the electrodeposition of zinc on ametal component from an electrolyte according to claim
 1. 13. The methodaccording to claim 12, wherein the metal component comprises or consistsof iron or an iron alloy.
 14. The method according to claim 12, whereindeposition occurs at a temperature of 20 to 50° C., preferably 25 to 40°C., and a current density of 0.2 to 10 A/dm², preferably 0.5 to 6 A/dm²,is used for the deposition.
 15. The method according to claim 12,wherein after the electrodeposition the component having a zinc coatingis subjected to a passivation treatment.
 16. The method according toclaim 12, wherein after the electrodeposition the component having azinc coating is annealed, optionally before or after a passivationtreatment.